The Multiphase Intracluster Medium in Galaxy Groups Probed by the Lyα Forest

The case is made that the intracluster medium (ICM) in spiral-rich galaxy groups today probably has undergone much slower evolution than that in elliptical-rich groups and clusters. The environments of proto-clusters and proto-groups at z > 2 are likely similar to spiral-rich group environments at lower redshift. Therefore, like the ICM in spiral-rich groups today, the ICM in proto-groups and proto-clusters at z> 2 is predicted to be significantly multiphased. The QSO Lyα forest in the vicinity of galaxies is an effective probe of the ICM at a wide range of redshift. Two recent observations of Lyα absorption around galaxies by Adelberger et al. and by Pascarelle et al are reconciled, and it is shown that observations support the multiphase ICM scenario. Galaxy redshifts must be very accurate for such studies to succeed. This scenario can also explain the lower metallicity and lower hot gas fraction in groups. Subject headings: galaxies:clusters:general — intergalactic medium — quasars:absorption lines